This invention relates in general to the conversion of coal to liquid fuels and more specifically to the removal of suspended mineral matter from coal-derived liquids. For purposes of this invention, coal-derived liquids are defined as the carbonaceous liquid product resulting from the liquefaction of anthracite, bituminous and sub-bituminous coals, lignite, oil shale, tar sands and other solid carbonaceous materials, regardless of the particular liquefaction process used. The suspended mineral matter includes unreacted or incompletely reacted carbonaceous material and inorganic material (normally termed "ash") which includes ironaluminum silicates, pyrites, etc.
A number of processes have been proposed in the prior art for removal of suspended mineral matter from coal-derived liquids. Normally much of the mineral matter is present as particles smaller than 10 microns in diameter. Such processes include sedimentation, filtration and centrifugation. In some instances, additives have been proposed to aid in separation. For example, U.S. Pat. No. 3,852,182 discloses the addition of a "promoter" consisting of a medium boiling fraction of the liquefaction product. In U.S. Pat. No. 3,790,467 solids separation is enhanced by adding both high and low boiling product fractions. In U.S. Pat. No. 3,607,718, unfiltered coal-derived liquid is passed into a settler through a heavy sludge layer to agglomerate particles. In U.S. Pat. No. 3,084,118, a two-component additive comprising a hydrocarbon and sulfuric acid is used.
A number of coal liquefaction processes involve molten salts as hydrogenation catalysts. Such processes typically involve introducing finely ground coal into a continuous phase molten salt at elevated temperature and hydrogen pressure. The gaseous and normally liquid products and unreacted hydrogen are flashed off leaving a molten salt phase and residual liquid. In some such processes ash has been observed to accumulate in the molten salt, causing partial deactivation. See, for example, U.S. Pat. Nos. 3,663,452, 3,790,468 and 3,844,928. In U.S. Pat. No. 3,663,452, coal was mixed in an autoclave under H.sub.2 pressure with a relatively large volume of molten salt catalyst. The resulting liquid product was said either to contain no ash or to be readily separable from the ash. In the prior art molten salt processes the primary purpose of the molten salt was to function as a hydrogenation catalyst and as a transport media for coal.